I had the privilege of attending a talk by Luxology where they detailed some of their internal research on CPU and GPU rendering technologies, finding GPU rendering approaches surprisingly disappointing in comparison. He makes a great comparison between CPUs and GPUs, stating that GPUs are very 'Wide' but computationally 'Shallow', in contrast to CPUs which are very narrow but computationally deep.

They've released the presentation as a Quicktime video showing their results. I highly recommend you check it out and see their results. I have some issues with their findings, but it's a great comparison that shows that GPUs are not a panacea for all problems. As he says, 'It's clear that the GPU is not the magic bullet they had hoped', and then moves towards heterogeneous solutions merging the best of both worlds. Later he shows some 'pure research' work with networked rendering using the BOXX renderPro system.